The draw tool allows to draw shapes and write text outside of the original image boundaries. Basically this is not a bad thing, but upon saving all content outside of the original image boundaries is lost without warning.

I suggest to either restrict the drawing area to the current image boundaries (probably the easier solution), or to auto-resize the canvas upon saving to also save all content outside the original image boundaries when necessary.

Additional note:
The draw tool does not span the entire available screen space when in Fullscreen mode. As you can see in the screenshot above, parts of the image from the Fullscreen view can still be seen at the bottom of the Draw tool screen. This does not happen when accessing the Draw tool from Viewer mode.